1. Field of the Invention
This invention is related to the technology and equipment for stressed metal skin sheet coverings, and especially for the technology and equipment of stressed skin sheet coverings for buses and railway rolling stock.
2. Description of Related Art
At present there are three known methods for applying stressed skin sheet coverings. The first method is a mechanical stretching, i.e. of welding the steel sheet as a skin cover onto the frame of a passenger vehicle under mechanical stretching.
By this technology the equipment needed for stretching is heavy, expensive and inefficient. It requires two processes: The initial and final stretching processes, and the stress distribution is not uniform. The second method uses electromagnetic induction heating for stretching. In this process the electricity consumption is also very large and the heating is not uniform. The third method presently applied uses a heating box for baking the steel skin sheet (see Chinese Patent No. CN 85101016A). Similar to the above mentioned technologies, the electricity consumption is very large, the heating is not uniform, and the operation is quite inconvenient.
The present invention uses the method of heating by the direct introduction of electricity into the steel skin sheet. Under the heating action of the electrical current the steel skin sheet is heated and stretched on its own, and then fixed by welding while it is still hot. After cooling, the residual stresses in the horizontal and vertical directions of the plane of the steel sheet have already been developed, so as to obtain a good result of skin covering.
Compared with the previous techniques the present invention provides obvious advantages. First of all, according to the technology of this invention, simple equipment is used, a reduction in cost is achieved, and there is a resulting convenience in operation and long service life. In addition, the electricity consumption is reduced compared to other methods. For steel sheets of 10 m in length, 0.6-0.7 m in width and 1-1.2 mm in thickness the time needed for heating the skin sheet will be no longer than 5 min, and the electricity consumption is no more than 3 kw-hr. For steel sheets of 23 m in length, 0.85 m in width and 1.5-2.5 mm in thickness, the heating time is no more than 25 min and the electricity consumption is no more than 40 kw-hr. Furthermore this invention can be applied to the technology of skin sheet coverings for various vehicles such as a side wall skin sheet covering of railway rolling stock, buses, subway cars, refrigerator cars and so on.